The present invention relates to a novel, wholly aromatic polyester which can be subjected to melt-polymerization and to melt-molding and is excellent in mechanical properties.
Recently, the demand for the material excellent in rigidity, heat-resistance and resistance to chemicals and usable for preparing anything such as fibers, films and molded articles has been raised. Although polyesters are broadly recognized as the material for preparing the general molded articles, polyesters have not been suitable for the use requiring a high strength, because of poor mechanical properties such as flexural modulus. In order to improve the mechanical properties of polyesters, a method for blending a reinforcing agent such as calcium carbonate, glass fibers, etc. with a polyester has been known. However, after having been blended, the density of the thus blended material becomes too large to reduce the merit of the plastic material, that is, the lightness in weight, and further, in the time of molding, the abrasion, etc. of the molding machine is very severe thereby causing the practical problems.
As a polyester which does not need any reinforcing agent and is suitable for use in applications requiring a high strength, liquid crystalline polyesters have been attracted attention in recent years. Since the time when W. J. Jackson published a thermally liquid crystalline high polymer comprising polyethylene terephthalate and hydroxybenzoic acid in "Journal of Polymer Science", Chemistry Edition, Vol. 14, page 2043 (1976), such a specified polyester has been attracted one's attention particularly. In his publication, Jackson reported that the liquid crystalline high polymer exhibited a rigidity of more than 5 times, a strength of more than 4 times and an impact strength of more than 25 times that of polyethylene terephthalate, thereby demonstrating the possibility of obtaining a resin of high performance characteristics. Then, the development of liquid crystalline polyester have been continuously conducted while aiming at the coexistence of the improvement of strength and rigidity and the melt-moldability as seen in Japanese Patent Applications Laying-Open No. 53-65421, No. 54-50594, No. 55-21491, No. 55-50022 and No. 55-106220. However, in spite of the proposals of more than 100 kinds of liquid crystalline polyester, no successful polyester as the material for molded articles is offered. This is due to the high orientability of the polymer in a molten state thereof resulting in a high anisotropy of the mechanical properties.
The present inventors, as a result of their studies for reducing the anisotropy of the mechanical properties of the liquid crystalline polyester, have attained the present invention.